1. Field of the Invention
This invention relates generally to the art of seismic prospecting and more particularly to an improved means and method for recording reflected seismic signals.
2. Description of the Prior Art
In reflection seismic exploration, field techniques are utilized to enhance the reflection signal with respect to the unwanted signal and with respect to the noise within the system components. In most reflection seismic field recording a spatial/wavenumber filter (x/k) is formed with either or both source arrays or receiver arrays. Often the data are time/frequency filtered (t/w) in the electronic instrumentation before recording. A typical receiver array formed in accordance with the prior art consists of geophones of a single natural frequency usually in the range of 8-14 Hz. The overall array length is made equal to or greater than the longest wavelength of the anticipated noise. In this manner it is hoped that most noise frequencies will be eliminated. For example an array of this character for P-wave propagation could be 200-400 feet in length and for shear wave work about 900 feet. The difficulty with this technique is that the high frequency response of the geophones to desired reflections is also so attenuated that it is beyond the dynamic range of typical analog-to-digital converters. The longer the array, the greater the attenuation of wanted high frequency components of the reflected signal.
The effect described above is not noticeable in the 10-50 Hz range of seismic data currently used in petroleum exploration. However, broad-band signals are highly desirable in order to maintain identifiable signal characteristics from reflection coefficient series while maintaining the higher frequencies for resolution of thin beds. See in this connection the figure and accompanying discussion on page 231 of Waters, Reflection Seismology, copyright 1978, John Wylie and Sons. This reference shows that a reflected pulse increases in discreteness or "definition" in direct proportion to increase in band width, a highly useful attribute in defining the presence of closely spaced layers.
In the newly developing art of reflection seismology applied to mineral exploration there has also been a failure to recognize the importance of broad-band seismic data with concentration only on high frequencies.
Given the limited digitization precision of present recording systems for recording broad-band signals, it is necessary to utilize some form of frequency dependent amplitude shaping to counteract attenuation of higher frequencies by the earth and to provide signals of all desired frequencies within this limited digitization precision.
It is therefore a general object of this invention to provide an improved means and method for enhancing the signal-to-noise ratio of broad-band signals in reflection seismic exploration.
It is a more specific object of this invention to provide an improved means and method for frequency-dependent amplitude shaping of broad-band seismic signals.
Other objects and advantages of this invention will become apparent from a consideration of the detailed description and drawings to follow taken in conjunction with the appended claims.